The present invention relates to particle sizing. It finds particular application in conjunction with two-phase flows and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other like applications.
Two-phase flows are used in many fuel combustion processes such as those found in gas turbine combustors, coal furnaces for power generation, and diesel engines. The efficiency of two-phase flows directly impacts the efficiency of the associated combustion process. For many industrial and fossil fuel energy processes, both the droplet/particle size and spatial distribution are of interest. The useful diagnostic tools should be able to make in situ measurements without disturbing the flow field so that the measurements can be meaningful for understanding these flows. This suggests the need for a novel non-intrusive optical technique that can provide instantaneous measurements of particle size and velocity at multiple spatial points in planar (2-D) fields so that estimates of the mass flow are obtained.
Digital Particle Image Velocimetry (DPIV) is a technique for obtaining planar measurements of particulate seeded flow fields. Light from a pulsed laser is formed into a thin sheet to illuminate a planar cross section of a flow. A CCD camera is used to record the light scattered by the particles at the two (2) instants that the light sheet is pulsed. The fluid velocity is determined by analyzing the recorded particle image data. Typically, cross-correlation data analysis is used to reduce the recorded image data to determine the fluid velocities. In most instances, the flow field is artificially seeded with tracer particles in order to measure the flow velocity. In two-phase flows, the second phase material provides the scattering sites for estimation of the flow velocity, or at least the velocity of the second phase. The objective of this work is to determine the feasibility of estimating particle size from Particle Image Velocimetry (PIV) image data. Particle size information can most likely be extracted provided the imaged particle size exceeds the optical system blur circle. The other major factors affecting the accuracy to which particle size information can be extracted are the optical system f/number, image system pixel resolution and dynamic range, the optical properties of the particles, and the characteristics of the scattered light from the particles.
Many different optical techniques for making in situ measurements of particle/droplet sizes are known.
An in-line holography system has been used to record holographic images of the light scattered from particles in a fluid. The hologram has been shown to be that of a screen with a circular aperture representing the particle image. Holograms have been reconstructed by passing a laser beam through the hologram and recording the reconstructed image at an on-axis observation plane. The reconstructed image contains the Fraunhofer pattern of the imaged particles. The size and shape of the particle is then determined by direct observation of the reconstructed image. Holography has been easily extended to velocity measurement by using double exposure holograms and analyzing the double aperture type of fringe structure in the reconstructed image to calculate the particle separation.
Laser interferometry has been used to characterize particles using the spatial frequency of the far-field fringe pattern in the forward-scatter region. Ovryn 4 uses partially coherent light and forward scattering to obtain Poisson Spot images of particles suspended in a solution to determine their 3-D velocities. Although not a sizing technique, this work illustrates the information content in the diffraction rings surrounding coherently illuminated particles and such rings are used to determine additional properties about the particle. The difficulty with these holographic and interferometric techniques lies in the requirement of a high spatial resolution detection media, typically photographic film.
Phase Doppler Particle Analysis (PDPA) is an existing technique that is able to make point wise velocity and size measurements simultaneously. Droplet sizes are obtained using PDPA, which relates the droplet size to the phase shift of light refracted through the drop and scattered to different positions on the receiving lens. The technique known as Laser Doppler Anemometry (LDA), which is a technique for obtaining point wise velocity measurements, has been modified for applications to particle sizing as well. The particular modifications relate the particle diameter to the LDA signal visibility. Other variations of the technique relate either the scattered intensity or the phase shift of the scattered light to the particle diameter.
In diffraction-based sizing techniques the particle interaction with the illumination is assumed to be analogous to the interaction of the same illumination with a uniformly illuminated circular aperture. The inherent problem in diffraction methods lies in the assumption that a particle acts as a uniformly illuminated aperture, which ignores many other scattering effects present in light scattering from a spherical particle, such as specular reflection, interference and refraction effects. Therefore, the Mie scattered signal has been used for determining particle characteristics.
The technique of Laser Sheet Dropsizing (LSD) has been used to determine droplet diameters. LSD uses the Mie signal, which is proportional to the particle diameter squared, and the Laser Induced Fluorescence (LIF) signal, which is proportional to the particle diameter cubed, for determining particle size.
Mie scattering has been investigated from small particles in the regime where the particle image is dominated by diffraction to determine the minimum particle diameter that is detectable for use in pulsed laser velocimetry techniques. Taking the Fourier transform of the Mie scattered electric fields from spherical particles results in multiple peaks in the intensity profile, the characteristics of which can be related to the particle size.
The ratio of the projected area of the reflection glare spot to the cross-sectional area of the droplet has been found to be proportional to the square of the ratio of the aperture radius to the distance from the droplet to the sensor surface. The signal to noise ratio of the light scattered from a spherical water droplet of known size is used as a measure of the ratio of light scattered from the glare spot to light scattered by the other part of the sphere surface. This information is then used to predict the shape and size of the reflection glare spot. Hence, information contained only in the reflection glare spot has been related to the droplet size. However, until now particle sizes (diameters) have not been determined as a function of the distance between a reflection glare spot and a transmission glare spot.
The present invention provides a new and improved apparatus and method which overcomes the above-referenced problems and others.